1. Field of the Invention
The present invention relates to apparatus and methods for forming multicolor images electrophotographically and more particularly to such apparatus and methods having improved structures and procedures for providing color-corrected reproductions.
2. Description of the Prior Art
Much technical effort has been directed toward developing apparatus and methods for producing high quality color reproductions electrophotographically. One common approach for such effort has been to form constituent color-separation electrostatic images (e.g. by exposing separate photoconductor sectors to a color original respectively through red, green and blue filters), to develop the electrostatic images respectively with different color toner (e.g. cyan, magenta and yellow toner) and then to successively transfer the different toner images in register onto a copy sheet.
One very difficult problem encountered in the above and other electrophotographic color imaging approaches is the correction for unwanted light absorptions of the reproducing system's colorants. For example it is well known that cyan pigments and dyes used in toners often have unwanted green and blue light absorptions (in addition to their desired red light absorption). Similarly, magenta toner often has significant unwanted blue light absorption (in addition to its desired green light absorption). If not corrected for, such unwanted toner absorptions can cause degradation in fidelity of the reproduction's color saturation and hue, as well as a darkening of the copy colors.
A related problem is that of exposure error of the red, green and blue information in the original color document being copied. There are usually side absorptions in the dyes, inks or toners used in the original. When the color-separation filter system is not precisely matched for a particular input colorant set, the amount of cyan, magenta and yellow toner produced in the copy will not be precisely proportional to the amount of cyan, magenta and yellow colorant in the original. For example, the amount of yellow toner produced in the copy will include an amount developed in proportion to the amount of yellow colorant in the original plus amounts developed in proportion to the amounts of cyan and magenta colorant in the original weighted by their respective blue absorptions within the pass-band of the blue color-separation filter which is used. Imperfect matching of the blue filter to the input colorants can cause these amounts of developed toner to differ from the amounts of input colorants in the original, thereby degrading the saturation and hue fidelity of the copy relative to the original.
A variety of solutions have been suggested for "color correcting" for unwanted light absorptions of the output toner colorants. For example U.S. Pat. Nos. 3,615,391; 3,836,244 and 3,844,783 disclose color-correction techniques wherein an element bearing an electrostatic mask pattern is placed into facing relation with a photoconductor sector which bears an electrostatic color-separation image and development occurs with the two electrostatic patterns competing for toner. These techniques involve additional steps (e.g. the formation of the otherwise unutilized mask pattern), are difficult to control accurately and are hard to implement in an automated machine.
Alternatively, an approach similar to graphics arts masking can be used. This involves forming a negative masking toner image and exposing the electrostatic color-separation image to the original through the masking image. Again additional steps are involved and the approach is difficult to implement in an automated machine.
U.S. Pat. No. 4,236,809 suggests performing color correction of a color-separation electrostatic image by selectively discharging it with a scanning laser beam (controlled in accordance with an electrical signal obtained from a previous electro-optic scanning of the original). U.S. Pat. No. 4,090,876 discloses a device using first and second ion modulating screens to form and color correct electrostatic color-separation images. Both of these latter approaches involve complex and expensive equipment additions to the electrophotographic apparatus, with the inevitably coupled problems in maintenance and reliability.